Purification of h2se

ABSTRACT

Disclosed are methods of purifying H 2 Se by removing H 2 S and H 2 O.

BACKGROUND

Hydrogen selenide (H₂Se) is a gas currently used in the photovoltaic industry for Copper Indium Gallium Selenide (CIGS) and Copper Indium Gallium Selenide Sulfide (CIGSS) thin film solar cells. Impurities in the H₂Se gas may negatively affect the deposition process of the Se-containing films. Two such impurities include H₂O and H₂S. H₂O may contribute oxygen to the metals being added to the film and hinder proper deposition. H₂S may be added in a controlled manner later in the process. However, sulfur is known to change the bandgap structure of the selenium-containing adsorber layer, thus impurities such as H₂S in H₂Se may affect the film performance. The combination of H₂S and H₂O is known to cause corrosion of distribution systems and their components, leading to component/system failure as well as introduction of metal impurities during the deposition process resulting from corrosion of the distribution system.

Prior art gas purifiers are known in the art. See, e.g., the Nanochem® C/CL-series gas purifiers from Matheson Tri-Gas, the Mini Gaskleen® gas purifier from Pall Microelectronics, and the MC-190 and HP-190 MicroTorr purifiers from SAES Pure Gas, Inc. However, none of these purifiers identify H₂Se as the gas to be purified and H₂O and H₂S as the impurities to be removed.

U.S. Pat. No. 4,797,227 to Tom et al. discloses a process and composition for purifying H₂Se and H₂Te to remove moisture and oxidant impurities therefrom. Tom et al. note that H₂Se and H₂Te are extremely difficult to purify because of their high reactivity and toxicity (col. 1, lines 35-37). Tom et al. utilize aluminum-containing compounds to remove water and oxidants from H₂Se or H₂Te (claim 1). The water and oxidants react with the aluminum-containing compounds (col. 5, lines 25-30). Tom et al. do not address the removal of H₂S from H₂Se.

JP2007-246342 to Taiyo Nippon Sanso Corp discloses a method for producing H₂Se which has high purity and in which the water content and the amount of H₂S are made particularly small. Taiyo Nippon Sanso Corp discloses the reaction of high purity H₂ with metallic Se and liquefies or solidifies the resulting H₂Se product. This process requires a lot of energy for both the reaction and the subsequent liquefaction/solidification.

A need remains for an efficient method to remove both H₂O and H₂S from available H₂Se supplies.

SUMMARY

Disclosed are methods of purifying a H₂Se-containing gas. The amount of H₂S in the H₂Se-containing gas is reduced below 10 ppmv by passing the H₂Se-containing gas through an AW-500 molecular sieve. The disclosed methods may further include one or more of the following aspects:

-   -   the purified H₂Se-containing gas containing less than 5 ppmv         H₂S;     -   reducing an amount of H₂O in the H₂Se-containing gas below 20         ppmv by passing the H₂Se-containing gas through a 4A molecular         sieve;     -   the purified H₂Se-containing gas containing less than 1 ppm H₂O;     -   passing the H₂Se-containing gas through the 4A molecular sieve         prior to passing it through the AW-500 molecular sieve; and     -   receiving the purified H₂Se-containing gas in a cylinder cooled         to a temperature ranging from approximately −20° C. to         approximately −60° C.

Also disclosed are methods of purifying H₂Se by passing H₂Se through a 4A molecular sieve and subsequently passing H₂Se through an AW-500 molecular sieve. The disclosed methods may further include one or more of the following aspects:

-   -   the purified H₂Se containing less than 10 ppmv H₂S;     -   the purified H₂Se containing less than 5 ppmv H₂S;     -   the purified H₂Se containing less than 20 ppmv H₂O;     -   the purified H₂Se containing less than 1 ppm H₂O; and     -   receiving the purified H₂Se in a cylinder cooled to a         temperature ranging from approximately −20° C. to approximately         −60° C.

BRIEF DESCRIPTION OF THE FIGURES

For a further understanding of the nature and objects of the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying figure wherein:

FIG. 1 a is an illustration of an exemplary purification container;

FIG. 1 b is an illustration of an alternate purification container;

FIG. 2 is an illustration of an exemplary purification system; and

FIG. 3 is an illustration of a second purification system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Disclosed are methods of purifying H₂Se-containing gases, such as “pure” H₂Se gas. Applicants have discovered that the quantities of impurities in H₂Se may change during use or over time. For example, a H₂Se cylinder initially containing 25 ppm H₂O contained 600 ppm H₂O when the cylinder was nearly empty. Of course, these changes will affect the quality of the resulting photovoltaic product. Therefore, H₂Se used for CIGS or CIGSS deposition should have as high a purity as possible.

Due to the reactivity of H₂Se and H₂Se-containing gases, the disclosed purification methods and any subsequent storage should be performed in systems and stored in vessels through which inert gas previously flowed to remove air and moisture. Exemplary inert gases include N₂, Ar, He, and combinations thereof. In the following examples, N₂ was used. The stainless steel components of the purification system should undergo complete electro polish and drying.

In order to prevent generation of additional impurities in the purified H₂Se gas, it should be stored in cylinders having surfaces with which the H₂Se does not react. Suitable surfaces include steel, carbon steel with Ni coating, aluminum, stainless steel, glass, quartz, nylon, metal alloys sold under the trademark Monel®, metal alloys sold under the trademark Hastelloy®, fluorine-containing resins sold under the trademark Teflon®, synthetic rubber sold under the trademark Kalrez®, synthetic rubber sold under the trademark Viton®, and/or glassware sold under the trademark Pyrex®. Preferably, the purified H₂Se is stored in cylinders made of aluminum or carbon steel with Ni coating.

H₂Se-containing gas is commercially available. H₂Se is most often produced in a high temperature reaction of H₂ with molten Se [H₂ (g)+Se (molten)→H₂Se (g)], The reaction produces a H₂Se-containing gas comprising approximately 95% v/v to approximately 99.9% v/v H₂Se, and typically comprising approximately 99.5% v/v to approximately 99.99% v/v H₂Se. H₂S impurities in the H₂Se-containing gas are produced from sulfur impurities in the selenium used to produce the gas. While low-sulfur selenium is commercially available, it is very expensive to remove sufficient sulfur to produce low H₂S in the H₂Se-containing gas.

Applicants have surprisingly found that H₂S in the H₂Se-containing gas is adsorbed by AW-500 when flowing the H₂Se-containing gas through AW-500 molecular sieve. This result is unexpected because a different molecular sieve having the same pore size as the AW-500 molecular sieve did not remove H₂O or H₂S from the H₂Se-containing gas.

H₂Se has a critical diameter of approximately 4 angstrom (146 pm). The critical diameter is the approximate molecular diameter of a molecule. H₂S has a critical diameter of 3.6 angstroms (134 pm). H₂O has a critical diameter of 3.2 angstroms (95 pm). Based on these sizes, one of ordinary skill in the art would expect the best molecular sieve that would capture H₂O and H₂S without capturing H₂Se to have a pore size of approximately 4 angstroms. However, the AW-500 molecular sieve has a pore size of 5 angstroms, but still effectively removes H₂S (3.6 angstroms/134 pm) from H₂Se (4 angstroms/146 pm), notwithstanding that both molecules are smaller than the pore size of the AW-500 molecular sieve. Additionally, the polarities of the three molecules are similar: H₂Se (0.59 D), H₂S (0.97 D), and H₂O (1.85 D), making separation by adsorption techniques difficult (D=debyes; 1 coulomb-meter=2.9979×10²⁹ D).

The H₂Se-containing gas contacts the AW-500 molecular sieve by flowing through a container of the AW-500 molecular sieve. The AW-500 molecular sieve is commercially available. Currently, AW-500 is supplied as 1/16″ or ⅛″ pellets. Either size may be used in the disclosed methods. The AW-500 molecular sieve may be placed in a container made of stainless steel. Alternatively, the AW-500 molecular sieve may be placed in a container made of aluminum, stainless steel passivated by SiH₄, or nickel plated carbon steel. The container should have an inlet and outlet. In one alternative, depicted in FIG. 1A, the inlet 10 and outlet 20 are located on opposite ends of the container 1 in order to minimize dead flow zones within the molecular sieve. The arrows in FIG. 1A depict the direction of gas flow through the container 1. However, alternative inlet and outlet locations may also be desired, such as the inlet 10 being located on one end of a cylindrical container 1 with the outlet 20 located on the cylindrical surface of the container 1 at the other end of the container 1. One of ordinary skill in the art will recognize that the shape of the container 1 is not limited to a cylinder, but may also include spherical, cubic, or other polyhedric shapes.

The commercially available AW-500 molecular sieve is provided in its activated state by the manufacturer. Applicants, however, re-activate the AW-500 molecular sieve prior to use in order to remove any contaminants that may have been adsorbed during transport from the manufacturer, during transport from its original container to the purification container, or from the purification container itself. Applicants activate the AW-500 molecular sieve by heating it to a temperature ranging from approximately 150° C. to approximately 250° C. under an inert gas for approximately 10 to approximately 24 hours. The inert gas may be N₂, Ar, He, or combinations thereof. One of ordinary skill in the art will recognize that the amount of molecular sieve to be activated will determine the activation time and temperature, with larger quantities requiring either longer times or higher temperatures and smaller quantities requiring less time or lower temperatures. The AW-500 may be activated by heating to temperatures as high as 600° C. However, heating the AW-500 to such temperatures will require the remaining components in the purification system, such as valves, to become more expensive because they will be engineered to withstand exposure to such temperatures. As a result, activation at temperatures below 300° C. permits the use of less expensive valves in the purification system.

The initial reaction between the H₂Se-containing gas and the AW-500 molecular sieve produces CO₂. Therefore, after activation of the AW-500 molecular sieve, the initial H₂Se-containing gas flowing through the AW-500 molecular sieve is vented. Applicants have not calculated the time required to reduce the CO₂ levels resulting from initial contact between the H₂Se-containing gas and the AW-500 molecular sieve with relation to the amount of molecular sieve. However, the initial reaction causes the temperature of the AW-500 molecular sieve to rise. In order to produce high purity H₂Se gas (i.e., containing low quantities of CO₂), Applicants have allowed the H₂Se gas to flow through the AW-500 molecular sieve until the temperature of the AW-500 molecular sieve returned to room temperature (approximately 15° C. to approximately 30° C.). For approximately 10 kg of AW-500 molecular sieve and a H₂Se-containing gas flow rate of approximately 100 cc/min to approximately 1,000 cc/min, the average time to reach room temperature has been approximately 2 hours. The purified H₂Se gas contained less than approximately 3 ppm CO₂. One of ordinary skill in the art will recognize that different amounts of AW-500 may require longer or shorter venting periods. Additionally, one of ordinary skill will recognize that higher concentrations of CO₂ may be acceptable for some H₂Se applications and, as a result, the disclosed purification process may require little or no venting time.

In order to reduce the amount of H₂S in the H₂Se-containing gas, the H₂Se-containing gas flows through the AW-500 molecular sieve that has been prepared in accordance with the paragraphs above. The H₂Se-containing gas should remain in contact with the AW-500 molecular sieve for approximately 0.5 seconds to approximately 100 seconds, and preferably from approximately 2 seconds to approximately 40 seconds. The process may take place at room temperature (approximately 15° C. to approximately 30° C.). Alternatively, the process may take place at a temperature ranging from approximately 0° C. to approximately 35° C. However, as shown in the Examples that follow, more H₂S is adsorbed on the AW-500 molecular sieve during the room temperature process than during the lower temperature processes.

The resulting H₂Se-containing gas has less than 10 ppmv H₂S, and preferably less than 5 ppmv H₂S, and contains between approximately 99.995% v/v and approximately 99.999% v/v H₂Se.

Water (H₂O) is also preferably removed from the H₂Se-containing gas. The presence of moisture may contribute to the corrosion of piping and gas distribution systems even at very low concentrations. H₂O may be removed by flowing the H₂Se-containing gas through 4A molecular sieve. The 4A molecular sieve is commercially available. Currently, 4A molecular sieve is supplied as 14×30 granular, 10×20 beads, 8×12 beads, 4×8 beads, 1/16″ pellets, and ⅛″ pellets. Any of these sizes may be used in the disclosed methods. Like the AW-500 molecular sieve, the 4A molecular sieve may be stored in a stainless steel container. Alternatively, the 4A molecular sieve may be placed in a container made of aluminum, stainless steel passivated with SiH₄, or nickel plated carbon steel. Like the container for the AW-500 molecular sieve, the container for the 4A molecular sieve should have an inlet and outlet. In one alternative, depicted in FIG. 1A, the inlet 10 and outlet 20 are located on opposite ends of the container 1 in order to minimize dead flow zones within the molecular sieve. The arrows in FIG. 1A depict the direction of gas flow through the container 1. However, alternative inlet and outlet locations may also be desired, such as the inlet 10 being located on one end of a cylindrical container 1 with the outlet 20 located on the cylindrical surface of the container 1 at the other end of the container 1 as depicted in FIG. 1B. One of ordinary skill in the art will recognize that the shape of the container 1 is not limited to a cylinder, but may also include spherical, cubic, or other polyhedric shapes.

Prior to use, the 4A molecular sieve is activated by heating it under an inert gas to a temperature ranging from approximately 150° C. to approximately 250° C. for approximately 10 to approximately 24 hours. The inert gas may be N₂, Ar, He, or combinations thereof. N₂ is adsorbed on the 4A molecular sieve at these temperatures. Therefore, a He purge should follow activation of the 4A molecular sieve under a N₂ atmosphere in order to produce a purified H₂Se gas having low quantities of N₂. For each 500 g of 4A molecular sieve, the flow of the inert gas should be proportional to approximately 100 cc/min (i.e. 500 g=100 cc/min; 1 kg=200 cc/min, etc.). After approximately 8 hours of He purge, purification of the H₂Se-containing gas may begin. One of ordinary skill in the art will recognize that different amounts of 4A may require a longer or shorter activation period.

In order to reduce the amount of H₂O in the H₂Se-containing gas, the H₂Se-containing gas is passed through the 4A molecular sieve that has been prepared in accordance with the paragraph above. The H₂Se-containing gas should remain in contact with the 4A molecular sieve for approximately 0.5 seconds to approximately 100 seconds, and preferably from approximately 2 seconds to approximately 40 seconds. The process may take place at room temperature (approximately 15° C. to approximately 30° C.). Alternatively, the process may take place at a temperature ranging from approximately 0° C. to approximately 35° C.

The resulting H₂Se-containing gas has less than 20 ppmv H₂O, and preferably less than 1 ppmv H₂O, and contains between approximately 99.995% v/v and approximately 99.999% v/v H₂Se.

The water removal step using the 4A molecular sieve may occur before or after the H₂S removal step using the AW-500 molecular sieve. When the water removal step follows the H₂S removal step, Applicants believe that the 4A molecular sieve helps to reduce the CO₂ produced from the reaction of H₂Se and the AW-500 molecular sieve. When the H₂S removal step follows the water removal, Applicants believe that the water content in the H₂Se-containing gas will be reduced so that little to no CO₂ will be produced by the reaction of H₂Se and the AW-500 molecular sieve. In another alternative, both the AW-500 molecular sieve and the 4A molecular sieve may be stored in the same container to simultaneously remove H₂S, H₂O, and CO₂.

After purification, the purified gas may be transferred to suitable cylinders at a temperature ranging from approximately −20° C. to approximately −60° C. The cylinder may be located on a scale to assure that the cylinder is not overfilled with H₂Se.

One exemplary purification system is depicted in FIG. 1. The H₂Se-containing gas is contained in the cylinder labeled H₂Se Source. The cylinder valve (not shown) and valve V1 are opened allowing the H₂Se-containing gas to flow into Vessel-1 containing one of the disclosed molecular sieves in order to remove either H₂O or H₂S from the H₂Se-containing gas. Pressure sensor P1 indicates the H₂Se Source cylinder pressure. Pressure sensor P1 may be a pressure gauge or a pressure transducer. Valve V2 is opened to allow the H₂Se-containing gas to flow to Vessel-2 containing a different molecular sieve than contained in Vessel-1. For example, if Vessel-1 contains AW-500, Vessel-2 contains 4A. Alternatively if Vessel-1 contains 4A, Vessel-2 contains AW-500. In another alternative, both Vessel-1 and Vessel-2 may contain both AW-500 and 4A. The purified H₂Se then flows through valve V3 into the cylinder labeled H₂Se Purified (when the cylinder valve, not shown, is open). Pressure sensor P2 will give an indication of any unexpected pressure drop across Vessel-1 or Vessel-2. Pressure sensor P2 also gives an indication H₂Se gas is flowing to the H₂Se Purified product cylinder. Pressure sensor P2 may also be a pressure gauge or pressure transducer. Valves V1 and V2 may be closed if service to Vessel-1 becomes necessary. Valves V2 and V3 may be closed if service to Vessel-2 becomes necessary.

A second exemplary purification system is depicted in FIG. 2. The H₂Se-containing gas is contained in the cylinder labeled H₂Se Source. Valve V9 is used to assure the entire system is properly purged of any contaminants by cycling between (a) pressurized purge gas (e.g., N₂, He, Ar, or other inert gas) and (b) pulling a vacuum on the system. Whenever the source or product cylinder is connected or disconnected from the system, the purge/vacuum cycling would be used to maintain the purity of the H₂Se purification system. This procedure will also prevent any H₂Se exiting the system to expose the operator and prevent the entrance of air and other impurities into the system.

Before operation of the purification system all valves would be closed. The H₂Se Source cylinder valve (not shown) would be opened, followed by valves V1, V2, V4, V5, V6, V7, and V8. Vessel 1 may contain molecular sieve 4A and Vessel 2 may contain molecular sieve AW-500. Alternatively, Vessel 1 may contain molecular sieve AW-500 and Vessel 2 may contain molecular sieve 4A. In another alternative, both Vessel 1 and Vessel 2 may include both molecular sieve AW-500 and 4A. Pressures sensor P1 indicates the H₂Se Source cylinder pressure. Filter 1 and Filter 2 are used to prevent particulates from coming out of Vessel 1 and Vessel 2 and going into the purified H₂Se Product cylinder. Pressure sensor P2 will give an indication of any unexpected pressure drop across Vessel 1, Vessel 2, Filter 1 or Filter 2. Pressure sensor P2 also gives an indication that H₂Se gas is flowing to the H₂Se Product cylinder.

V10 may be used to send an H₂Se sample to analysis instrumentation to determine if the H₂Se is the proper quality to fill in the H₂Se Product cylinder. If the H₂Se quality is at the required level, cooling means together with the thermocouple TC may be used on H₂Se Product cylinder to reduce the pressure of the cylinder maintain a pressure differential between the source cylinder and the product cylinder. Suitable cooling means include liquid baths capable of cooling the H₂Se Product cylinder to temperatures as low as −60° C., such as liquid nitrogen, dry ice, acetone, isoproponal, and other known heat transfer liquids. The cooling means and TC act as a temperature controller to cool the product cylinder. This creates a pressure difference that enables flow of H₂Se from the H₂Se Source to the H₂Se Product. Once the H₂Se Product cylinder valve (not shown) is opened, high purity H₂Se will flow from the H₂Se Source cylinder to the H₂Se Product cylinder. Scale 1 will be able to indicate the amount of product that has left H₂Se Source cylinder and scale 2 will indicate the amount of product that has been filled into the H₂Se Product cylinder. Valves V4 and V5 may be closed if service to Purifier 1 and/or Filter 1 becomes necessary. Valves V6 and V7 may be closed when service to Purifier 2 and/or Filter 2 becomes necessary. Filter 3 is a final filter used to assure no particles enter the H₂Se product. If purification of the H₂Se source is not necessary, valves V2 and V8 may be closed and valves V1 and V3 opened.

EXAMPLES

The following non-limiting examples are provided to further illustrate embodiments of the invention. However, the examples are not intended to be all inclusive and are not intended to limit the scope of the inventions described herein.

Example 1

The following series of molecular sieves and catalysts were screened.

H₂O H₂O H₂S H₂S Purifiers challenge removal challenge removal A AW-300 60 0% 39.2 25% B AW-500 25 C. — NT 39.2 85% B AW-500 3 C. — NT 33.8 30% B AW500 −15 C. 690  0% 38.2 22% C 5A 690  0% 38.2 0% D 4A Pt—Si — NT 38.2 13% E ZnO 690  0% 38.2 61% F R3-12 3 × 5 mm 690  10%  38.2 38% G R3-12 0.5-1.0 mm 75 30%  45 47% H BASF COS 75 0% 45 28% J 4A 75 95%  45 36% NT—Not Tested

The H₂O and H₂S challenge levels in the table represent the amounts of H₂O and H₂S in ppmv that were in the H₂Se source. The H₂O and H₂S removal columns provide the % v/v of each that were removed from H₂Se.

The H₂O challenge and H₂O removal analysis was performed by a Meeco Aquavolt+ with a P₂O₅ electrolyte H₂O analyzer. However, Se deposited on the P₂O₅ electrolyte. Additionally, the response time was very slow. As a result, H₂O analysis was not performed in all of the tests in the table above. Additional H₂O analysis was performed with a HALO H₂O analyzer.

The H₂S challenge and the H₂S removal analysis was performed by GC HP 5890/PDHID. A Restek GC column capable of detecting N₂, O₂, CO₂, CH₄, and H₂S in the H₂Se-containing gas was utilized. The GC was calibrated using a gas standard containing 30 and 100 ppm of each of these impurities. The detection limits were ˜0.1 ppm (3×S/N).

In addition to measuring the removal of the H₂S and H₂O impurities, the generation of other impurities due to contact with H₂Se was analyzed.

While ZnO successfully removed H₂S, it seemed to produce another impurity that has not yet been identified.

Molecular sieve 5A did not remove H₂O or H₂S. This may be the result of H₂Se adsorption in the 5 angstrom pores.

AW-500 removed H₂S while releasing some CO₂. Surprisingly, AW-500 removed H₂S from H₂Se, even though AW-500 has the same 5 angstrom pore size as molecular sieve 5A. Capacity of the AW-500 molecular sieve for H₂S in H₂Se was found to be >1% g H₂S/g AW-500.

The capacity of the 4A molecular sieve for H₂O in H₂Se was found to be 2.4% g H₂O/g 4A.

Example 2

One vessel containing 4A molecular sieve was used specifically to remove H₂O from H₂Se at ˜110 psig @ 72° F. The second vessel containing AW-500 molecular sieve, and in series with the first vessel, was used to remove H₂S from H₂Se at ˜110 psig @ 72° F. Analytical equipment capable of measuring both H₂S and H₂O concentrations in H₂Se was located downstream of the vessels. Measurement of the level of H₂O in H₂Se was below 1 ppm, and H₂S in H₂Se was below 5 ppm. At these levels the chance for reaction with H₂O or H₂Se in the process is quite low.

It will be understood that many additional changes in the details, materials, steps, and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above and/or the attached drawings. 

1. A method of purifying a H₂Se-containing gas, the method comprising reducing an amount of H₂S in the H₂Se-containing gas below 10 ppmv by passing the H₂Se-containing gas through an AW-500 molecular sieve.
 2. The method of claim 1, wherein the purified H₂Se-containing gas contains less than 5 ppmv H₂S.
 3. The method of claim 1, further comprising reducing an amount of H₂O in the H₂Se-containing gas below 20 ppmv by passing the H₂Se-containing gas through a 4A molecular sieve.
 4. The method of claim 3, wherein the purified H₂Se-containing gas contains less than 1 ppm H₂O.
 5. The method of claim 3, wherein the H₂Se-containing gas is passed through the 4A molecular sieve prior to being passed through the AW-500 molecular sieve.
 6. The method of claim 1, further comprising receiving the purified H₂Se-containing gas in a cylinder cooled to a temperature ranging from approximately −20° C. to approximately −60° C.
 7. The method of claim 3, further comprising receiving the purified H₂Se-containing gas in a cylinder cooled to a temperature ranging from approximately −20° C. to approximately −60° C.
 8. A purification method for H₂Se, the method comprising: passing H₂Se through a 4A molecular sieve; and subsequently passing H₂Se through an AW-500 molecular sieve.
 9. The method of claim 8, wherein the purified H₂Se contains less than 10 ppmv H₂S.
 10. The method of claim 9, wherein the purified H₂Se contains less than 5 ppmv H₂S.
 11. The method of claim 8, wherein the purified H₂Se contains less than 20 ppmv H₂O.
 12. The method of claim 11, wherein the purified H₂Se contains less than 1 ppm H₂O.
 13. The method of claim 8, further comprising receiving the purified H₂Se in a cylinder cooled to a temperature ranging from approximately −20° C. to approximately −60° C. 